New 750K guy

[chuck hahn]

Got a 79 CB750K and have been working on it a while.   Did the maintenance per a service manual I got . The manual is a Clymer manual by the way.  All the valves shims are within spec, I disassembled the carbs and did a 24 dip in berrymans carb sauce.  I also used the Morgan carb tune to sync the carbs.

It runs pretty darn good but i got a little problem accelerating.   Note this is while its running on the lift.   I am a bit confused on what mark i line up with the pointer at the top of the case.  I found a picture from google of the timing setup this bike has.   I currently have the pointer lined up with the right line of the double lines with the "S" to the left and the "F" underneath the double lines.

I thought this was the correct line which leads me to the problem.  If I roll the throttle on kinda easy..not a quick full throttle twist..it rev up nice and will go to 8,000 RPM smooth and easy.  Now the problem is that if I twist it real quick it wants to hesitate a bit and isnt as smooth and reactive like when i just easily roll on the throttle.   Am I making making sense trying to accurately describe whats going on??

If i have the pointer lined up right, my next thought was to bend the arm that pushes on the accelerator pump so thre is more of a gap.  My thinking was that maybe the accellerator pump is getting involved too early and kinda flooding the engine.

Think I should revisit the carb tune again and try resyncing the carbs yet again??   Ive worked on more than a few bikes but this Honda has got me fresh out of ideas where to check.  

[TOOLS1]

The line on the "F" is where you want the timing set to. I do not think your accelerator pump is the problem. What are you running for the breather, stock airbox or pods? Also, are you sure that the pilot and main jets did not get switched? And last but not least, you could have a vacuum leak.
TOOLS

[chuck hahn]

Thats what I am confused about Tools.  There is a T mark.  Then there is what looks like an "I" to the left of the T mark.  Then there is a set of double lines all the way to the left.   WHICH of these two double lines is the "F" line...right one or left one???

[chuck hahn]

Stock air box and filter.  Bike came with aftermarket Jardine exhaust from back in the early 80s far as i have been able to identify it.  Same ones as these on Ebay

https://www.ebay.com/p/NOS-Jardine-Honda-Cb750-Exhaust-Slip-on-Mufflers/863999530?iid=152993292215&chn=ps

EDIT.   I cant recall right now where the book said was the start point for the bottom mixture screws.   What is the consensus on where these should be set at??

[TOOLS1]

The "T" is for top dead center. It has been a long time since I worked on a DOHC 750 so I cannot say for sure that I remember how yours ls laid out. Could you post a picture? Mixture screws, I would say start with two turns out. Also, do you have a Honda Factory Service Manual? If not you can download it from our manuals thread.
TOOLS

[chuck hahn]

Got a Clymers Manual.   I tried to copy and paste a picture from Google Images but that didnt work.  It has the two black pickups..electronic ignition setup.  Not points in this bike.   I will try again on a picture.

[chuck hahn]

Did this work??   If so, this is what I see on the advancer.  I am assuming its the right line of the double set is the "F" mark.   My rotor tooth   and pulse tooth  line up though.  Even noticed they are in perfect alignment with the timing light going..both the 1/4 and 2/3 teeth are lined up when the engine is running.

[TOOLS1]

There should be another mark just like that 180* from that one for the other two cylinders. If both of them are lining between the two lines above the "F" at idle, you should be good. I think you have a carburetor problem.
TOOLS

[chuck hahn]

OK, so in referring to the double lines with the 'S next to them and the 'F' under them , The  pointer on the engine case points directly at the RIGHT line for both the 1/4 marks and the 2/3 marks.  I switched the timing light back and forth several times to verify that aspect.

So, whats been eliminated for certain:

The rotor tooth and the pickup tooths  line up exact across from each other when its running with where i have the timing set now...both for 1/4 and 2/3  pickups.
Valves are all withing specs..so no tight valves.
New advancer..bought a new NOS one off Ebay.
New spark units behind he battery box..no erradic flashes when timing light is on from either coil som its not an erradic spark issue.
Carbs had been dipped for 24 HRS in Berrymans carb and parts cleaner.  new accel pump and new air cutoffs installed when reassembled.
Carbs synced with my Morgan carbtune pro.
None of the slides seem to drag or act weird as far as easy to lift and they fall back down and dont hang up...none had any holes that i could find in the diaphrams.

So whats the missing link here?  Redip the carbs again and resync again?

[TOOLS1]

Could you have switched the pilot and main jets? Have you checked for vacuum leaks? Did you install new O-rings on the pilot screws? And what do you mean "none had any holes that i could find in the diaphragms. "? You should have metal slides that do not have a rubber diaphragm.
TOOLS

[chuck hahn]

Youre right..the manuals said to look at the edges for any scratches or damage to them..i guess Diaprams arent what they are called actually.  Metal with some machined in grooves around the rims. They set down on that nylon ring thing.  I even cleaned the hole inside the caps that the slide..guess that the proper name..goes up into to be sure there wasnt any crud in there.  Ysed carb spray and lots of Qtips to clean them out.

And yes there  were new orings for the screws in the kits.  Spring goes on, then the washer, then an oring.  Shot a little oil on orings so they wouldnt snag going in and tear   I will recheck the jets but as I recall i looked at each one and there was an obvious difference in hole sizes.  The bigger holes should be the mains and IM about 99.% sure they are right..but they will be rechecked to be positive.

I used a fine sire from a wire brush to poke all the holes in all the jets..and yes even down the center of the pilot jets, rinsed them in tons of carb spray, repoked again and more spray, then lots of compressed air. I learned the lesson on really getting those jets super clean already...LOL

[chuck hahn]

Tools..call me man.   Much easier than typing..

254-251-6288

[TOOLS1]

I am not allowed to use the phone. However Shiny is the guy you need to talk to, he is the DOHC guy.
TOOLS

[chuck hahn]

SO......After yanking the carbs yet again and rechecking everything for the unknown many times I could find absolutely nothing amiss.  Carbs back on and exactly the same so I remembered an old test a guy showed me about air filters and airboxes.

Took the filter and cover off and it ran even worse so i headed back to the garage.  Put the filter and the spring clip in but left the cover off and away i go again.  Ran like the day it was made without a hiccup.  Out cover back on and problem returned.

So what i think is its jetted a tad too big??? So the airbox cover adds just enough extra restriction fromm the intake side to make it run like its being choked out a little with the main jets that are in it now???   Does that make sense???

[TOOLS1]

That is interesting. Do you know what size jets are in it? And if they could have been drilled out at some time?
TOOLS

[chuck hahn]

No...I honestly didnt even look at the numbers on the mains.  They may have been drilled at some point in its life.  I was thinking that whenever they put those aftermarket Jardine pipes on they had to have fiddled with the jets.  Just an tad too fat thought for what the engine likes obviously.

I dont see any harm in just leaving the airbox cover off for now and   I was also pondering whether the supposed extra breathing ability of a K&N filter may solve the situation with the airbox cover installed.  What do you guys think about that idea???

[chuck hahn]

Paper filter in it now is the stock one and it doesnt look dirty at all to me.